This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7 119 from an application entitled ELECTROPHOTOGRAPHIC IMAGING SYSTEM AND METHOD TRANSFERRING IMAGE THEREOF earlier filed in the Korean Industrial Property Office on the Sep. 21, 1999, and there duly assigned Serial No. 99-40669.
1. Field of the Invention
The present invention relates to an electrophotographic printing apparatus for printing an image developed on a photosensitive medium, and an image transferring method thereof, and more particularly, to an electrophotographic printing apparatus having a structure so that differences in peeling forces of components contributing to image transfer can be maintained within predetermined ranges, and an image transferring method thereof.
2. Description of the Related Art
In general, an electrophotographic printing apparatus such as a laser printer scans a laser beam on a photosensitive medium to form a latent electrostatic image, develops the latent electrostatic image using developing units, and transfers the developed image to a paper by means of a transfer unit. Electrophotographic printing apparatuses can be classified as wet type or dry type according to the developer usable in the apparatus.
Referring to FIG. 1, a general wet type electrophotographic printing apparatus includes a photosensitive medium 10 traveling along a predetermined track, laser scanning units 21 for scanning laser beams on the photosensitive medium 10 to form a latent electrostatic image, developing units 20 for developing the latent electrostatic image on the photosensitive medium 10, for toners such as black (K), cyan (C), magenta (M) and yellow (Y), a drying unit 30 for drying a carrier covered on the photosensitive medium 10, a transfer unit 40 for transferring an image (I) from the photosensitive medium 10, after having been dried by the drying unit 30, to a paper (P).
The photosensitive medium 10 includes a photosensitive belt 11 as shown in FIG. 1, or a photosensitive drum (not shown), or the like. The photosensitive belt 11 travels along a predetermined track while looped around a driving roller 13, a transfer backup roller 15, and a steering roller 17. In the vicinity of the photosensitive belt 11, an eraser 3 for irradiating the photosensitive belt 11 with light to lower electric potentials distributed on the photosensitive belt 11 to a predetermined level, and a charger 5 for charging the photosensitive belt 11 to raise the potential of the photosensitive belt 11 lowered by the eraser 3 to a predetermined potential are installed.
The drying unit 30 includes a drying roller 31 for contacting the surface of the photosensitive belt 11 on which an image (I) is formed, and absorbing carrier thereon, and a regeneration roller 33 for heating the drying roller 31 so as to evaporate a carrier absorbed by the drying roller 31. Here, if the drying roller 31 peels off even a portion of an image (I) developed on the photosensitive belt 11, the quality of the image deteriorates.
The transfer unit 40 includes a transfer roller 41 which is disposed to face the transfer backup roller 15 with the photosensitive belt 11 interposed therebetween and to which an image (I) developed on the photosensitive belt 11 is transferred, and a fuser roller 43 disposed to face the transfer roller 41 while allowing a paper (P) to pass therebetween for fixing an image transferred to the paper (P). Here, an image transferred to the transfer roller 41 is transferred to the paper (P) fed between the transfer roller 41 and the fuser roller 43.
In the wet type electrophotographic printing apparatus configured as described above, whether or not a developed image is sequentially transferred from the photosensitive belt 11 to a paper (P) is determined by differences in the surface energies of the photosensitive belt 11, the drying roller 31, the transfer roller 41, and the paper (P). That is, since toner forming an image is transferred from one member to another having a larger surface energy than the former, materials of respective members are chosen in consideration of their surface energies.
Here, the surface energy of a member functions as a factor deciding a surface adhering force Fsurf of a toner particle, the surface adhering force Fsurf being defined by the following Formulas 1 and 2.
The surface adhering force Fsurf of a toner particle is expressed as Formula 1 based on Lifshitz-van der Waals equation, as follows:                                           F            surf                    =                                    ℏ              ⁢                              xe2x80x83                            ⁢              ω              ⁢                              xe2x80x83                            ⁢              R                                      8              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢                              z                2                                                    ,                            (        1        )            
where R is the radius of a toner particle, z is the distance between particles, and "Hslashed"xcfx89 is the surface energy of a particle.
When the value of the distance between particles z is a constant, a proportional expression shown in Formula 2 is satisfied, as follows:                                           F            surf                    R                ∝                  ℏ          ⁢                      xe2x80x83                    ⁢                      ω            ⁢                          xe2x80x83                        .                                              (        2        )            
Taking the above relation into consideration, the surface energy can be converted into a value in dyne/cm. Therefore, in the following description, a value of Fsurf/R (dyne/cm) is defined and used as a surface energy.
In addition, the surface energy defined as above is an absolute value of a selected material, and the value can be used in a useful manner if it can be measured directly, but it is very difficult to directly measure the surface energy.
On the other hand, as a method of indirectly measuring the surface energy, there is a method in which after a liquid of known surface tension is dropped on an object, the contact angle of the liquid is measured. This method was proposed by Thomas Young in 1805, and the method is well-known and referred to as Young""s equation.
However, when the surface energy is measured indirectly according to such a method, there can be the following problems. First, a point for measuring a contact angle must be determined, but there can be a difference of about 1xc2x0 to 2xc2x0 in the contact angle due to variations in the position of the measuring point. As a result, the typical deviation in a value of a surface energy is plus or minus (xc2x1) 2 dyne/cm which is generally too large a deviation. Second, since the indirect measuring method is performed on discontinuous points, measurements can be accomplished on sampled points and cannot be performed actually on the entire surface of a roller. Therefore, it is very difficult to apply this method to mass production. Third, at least two standard liquid samples for such indirect measurement are required, and it is often difficult to manage the standard samples. That is, the standard samples are kept in a controlled atmosphere at a predetermined temperature and humidity, in a unopened state.
Also, the surface energy can be determined by measuring and comparing the peeling forces (in gram force per inch (gf/inch)) of at least two components of different materials. In this regard, the peeling force is the force required to peel an adhesive tape attached to a component such as a transfer roller or a photosensitive belt, and is a relative value depending on the type of adhesive tape used for measurement, the pressing force applied during the attachment of the tape, the operation speed of the measuring apparatus, the ambient temperature, and the like.
Referring now to FIG. 2 is a perspective view of a peeling force measuring apparatus for describing a peeling force measuring method using the such apparatus. As shown in FIG. 2, the measuring apparatus 50, for example, an IMASS SP-2000 manufactured by Instrumentor Inc. includes a stage 51, and a load cell 53 for measuring a load.
After an object whose peeling force is to be measured, for example, a transfer roller 41xe2x80x2 is installed on the measuring apparatus, an adhesive tape 55, for example, 202 Masking Tape of 3M Corp., is taped on the surface of the transfer roller 41xe2x80x2. Then, after the load cell 53 is connected to the adhesive tape 55, the transfer roller 41xe2x80x2 is moved in a direction in which the load applied to the load cell 53 is increased. The peeling force is the load acting on the load cell 53 at the moment when a portion of the adhesive tape 55 is separated from the surface of the transfer roller 41xe2x80x2 by such movement of the transfer roller 41xe2x80x2.
Referring now to FIG. 3, by the above-described peeling force measuring method, when peeling forces in a photosensitive belt and a transfer roller were measured in constant conditions with a large amount of sheets of paper being printed, it was found that, as shown in FIG. 3, as the number of printed sheets of paper increased, the peeling forces also increased. That is, it was found that, as illustrated in FIG. 3, in the case of the transfer roller, while the peeling force was about 100 gram force per inch (gf/inch) when the transfer roller was in a clean state, the peeling force measured after 1,000 sheets of paper were printed was about 400 gram force per inch (gf/inch). On the other hand, it was found that, in the case of the photosensitive belt, while the peeling force was about 5 gram force per inch (gf/inch) when the photosensitive belt was in a clean state, the peeling force measured after 3,000 sheets of paper were printed was about 200 gram force per inch (gf/inch).
In the case of the transfer roller, it was determined that the reason why the peeling force was increased as described above, with reference to FIG. 3, was because the surface of the transfer roller was contaminated by the material of the photosensitive belt, remaining toner, positive charges included in transferred toner, counter ions, carrier, ozone, nitric acid, the material of a dryer roller, oil particles, or the like.
As the peeling force is increased as described above, with reference to FIG. 3, there can be a problem in that, in a conventional electrophotographic printing apparatus, as illustrated in FIG. 1, for example, at least one component among the photosensitive belt 11, drying roller 31, and transfer roller 41 must be replaced, or a bad transfer of an image can occur when the total printed sheets of paper reaches a certain number. In addition, there can be a problem in that a fed paper can get wound around the transfer roller and therefore may not get discharged due to the increase of the peeling force, and a paper jam can occur. Further, there can be a problem in that when the relative relation of the photosensitive belt 11, drying roller 31, transfer roller 41, and paper (P) are not appropriately set as to peeling forces, a bad transfer of an image can occur even at the initial operation stage of a new printing apparatus.
It is an objective, among other objectives of the present invention to provide an electrophotographic printing apparatus and an image transferring method thereof adapted to decrease the probability of a bad transfer of an image by setting relations between peeling forces of a photosensitive belt, drying roller, and transfer roller.
Accordingly, to achieve the above objective and other objectives of the present invention, there is provided an electrophotographic printing apparatus including a photosensitive medium, developing units for developing respective images and forming respective films on the photosensitive medium, a drying roller for drying image films formed on the photosensitive medium by the developing units while contacting the photosensitive medium and rotating, and a transfer unit for transferring an image from the photosensitive medium to a paper while contacting the photosensitive medium and rotating, whereby, for image transfer, the peeling force of the drying roller (PFD) is in a range of 0 gram force per inch (gf/inch)xe2x89xa6PFDxe2x89xa6300 gram force per inch (gf/inch), the peeling force of the photosensitive medium (PFO) is in a range of 0 gram force per inch (gf/inch)xe2x89xa6PFOxe2x89xa680 gram force per inch (gf/inch), and the peeling force of the transfer roller (PFT) is in a range of 200 gram force per inch (gf/inch)xe2x89xa6PFTxe2x89xa6500 gram force per inch (gf/inch).
In addition, in an electrophotographic printing apparatus including a photosensitive medium, laser scanning units for forming respective latent electrostatic images on the photosensitive medium, developing units for developing respective images corresponding to the latent electrostatic images on the photosensitive medium, and a transfer unit for transferring an image from the photosensitive medium to a paper by using the difference in the surface energies thereof, the apparatus further includes a peeling force adjusting means for adjusting the peeling forces of any of the photosensitive medium and the transfer unit so that the surface energy of the transfer unit can be maintained to be higher than that of the photosensitive medium.
Also, to achieve the above objective, and other objectives of the present invention, there is provided an image transferring method for electrophotographic printing apparatus, the electrophotographic printing apparatus including: a photosensitive medium; laser scanning units for forming respective latent electrostatic images on the photosensitive medium; developing units for developing respective images corresponding to the latent electrostatic images on the photosensitive medium, and for forming respective films on the photosensitive medium; a drying roller for drying image films formed on the photosensitive medium by the developing units while contacting the photosensitive medium and rotating; and a transfer unit for transferring an image from the photosensitive medium to a paper while contacting the photosensitive medium and rotating, the image transferring method including the steps of: adjusting the peeling force of the drying roller; adjusting the peeling force of the photosensitive medium; and adjusting the peeling force of the transfer roller.